1. Field of the Invention
This invention relates to supervisory circuits, and more particularly to a circuit for supervising an alarm circuit having a plurality of parallel-connected, continuously-conductive alarm devices.
2. Description of the Prior Art
Increasing functional demands increase the complexity of many modern circuits. Even long-known circuits are today more complex than at the time of their invention. Such increased complexity may result both from increasing the size of the circuit to provide more, similar functions and from decreasing the size of the circuit to provide the same function in a smaller space. An alarm circuit extending throughout today's larger buildings exemplifies the former type of increased complexity, while an integrated circuit exemplifies the latter.
This increase complexity makes difficult or impossible the manual supervision of the proper functioning of individual devices in the circuit. For example, an alarm circuit having but few alarm indicating devices cound be supervised by periodically activating the alarm and manually checking the operability of each alarm device. However, when a large number of such alarm devices are employed, manual supervision becomes impractical. Similarly, when discrete circuits are employed, it is practical to tap and test individual circuit devices of the circuit, but when integrated circuits are used, it is difficult to tap and test portions of the integrated circuit.
Therefore, automatic supervision of circuit operability is desirable. Many circuit supervisory systems have been developed, but have not been entirely successful, especially when used with circuits having a plurality of parallel-connected, continuously conductive devices. If the impedance of the individual devices is low, the collective impedance across a circuit of such devices is considerably low. In some cases, the impedance across the circuit may be comparable with the impedance of conductors which connect the devices in parallel so that a discontinuity in one of the devices is difficult to detect. Furthermore, the low impedance of the circuit may be within the range of expected impedance variations inherent in the devices themselves. For example, the impedance across some devices varies more than ten percent with a 30.degree.C. change in the temperature of the device. Similarly, corrosion or vibration of contacts of the device may vary the rest impedance of the contacts sufficiently to affect the impedance across a parallel circuit of such devices.
Alarm circuits exemplify these problems for a system supervising the continued operability of individual alarm devices in the circuit. Alarm circuits generally have a plurality of continuously-conductive, parallelconnected alarm devices such as bells, horns, lights or the like. Typically, thirty or more alarm devices may be connected in a given alarm circuit. Since the alarm circuit is operated only during an alarm condition, it is desirable to provide continual supervision of the operability of each alarm device in the circuit.
One known supervision system for an alarm circuit has an end of line resistance connected between conductors across which the alarm devices are parallel-connected. Diodes in series with each alarm device effectively open circuit the devices to a potential of one polarity applied to the conductors, rendering the alarm devices non-conductive and inoperable by the potential while permitting a potential of the opposite polarity to operate the alarm devices. the end-of-line resistance permits monitoring of the one potential to indicate the continuity of the conductors. Although conductor continuity indicates operability of the conductors in the alarm circuit, the operability of individual alarm devices in the circuit is not monitored by systems of this type.
As is reported in the literature, attempts have been made to employ operational amplifiers connected in a bridge circuit with the alarm circuit to supervise individual alarm devices in an alarm circuit. In such cases, it was found that the range of operating temperatures to which the alarm circuit was exposed resulted in unstable operational amplifier conditions. Furthermore, the low collective impedance of parallel connected alarm devices comprising the alarm circuit necessitated a bridge circuit so sensitive as to be uneconomically expensive or, if less expensive, unreliable.